1. Field of the Invention
The present invention relates to a dispersion compensating optical fiber and to a dispersion compensating optical fiber module. Specifically, the present invention relates to a dispersion compensating optical fiber compensating for wavelength dispersion and the dispersion slope caused by transmitting optical signals in an employed wavelength band selected from the wavelength region from 1.52 μm to 1.63 μm by means of a standard single mode optical fiber with a dispersion zero at 1.3 μm, and relates to a dispersion compensating optical fiber module including the dispersion compensating optical fiber.
2. Description of Related Art
Systems including optical amplifiers such as very-long-distance non-repeating relays for wavelengths of 1.52 to 1.63 μm are already commercially available as erbium-doped optical fiber amplifiers and are in practical use. Furthermore, the development of wavelength division multiplexing transmission has rapidly progressed with the increase in transmission capacity. Wavelength division multiplexing transmission system is already commercially available in a number of transmission lines. It is anticipated that the expansion of the wavelength band and the increase in the number of times wavelengths are multiplexed will rapidly progress in the future.
In order to perform high-speed transmission, the wavelength dispersion in transmission lines of optical fibers are preferably as small as possible in the transmission band, but are not zero. Moreover, in order to perform wavelength division multiplexing in a transmission system, it is important that the ratio of dispersion change relative to wavelength change in the entire length of the transmission path (which is referred to as “dispersion slope”, hereinafter) is small in order to decrease the difference in the dispersion between each wavelength in an employed wavelength band, in addition to decreasing amplification differences caused by the erbium-doped optical fiber amplifier in the employed wavelength band and decreasing the wavelength dispersion.
Moreover, recent long-distance systems require techniques for suppressing effects of nonlinearity which may cause deterioration, such as transmission characteristics, because the number of wavelengths multiplexed and the optical power propagating through the optical fiber rapidly increase.
The magnitude of the nonlinearity is expressed as n2/Aeff, where n2 represents the nonlinear refractive index for the optical fiber, and Aeff represents the effective area of the optical fiber. In order to suppress the effects of the nonlinearity, n2 is required to be small or Aeff is required to be large. However, it is difficult to significantly decrease n2 in an optical fiber composed of silica glass material, because n2 is a constant value of a material for the optical fiber. Therefore, it is important to increase Aeff in order to suppress the effect of nonlinearity.
Recently, single mode optical fiber networks for a wavelength of 1.3 μm are being used worldwide. When the transmission for a wavelength of 1.55 μm band is performed by using the optical fiber network, about +17 ps/nm/km of wavelength dispersion is caused. Therefore, when optical signals are transmitted through the optical fiber, the transmission characteristics are worsened by the wavelength dispersion.
A dispersion compensating optical fiber for compensating the wavelength dispersion has been developed and is commercially available. The dispersion compensating optical fiber has a large negative dispersion in the 1.55 μm wavelength band. By connecting the dispersion compensating optical fiber to a transmitting single mode optical fiber at a suitable length, accumulated positive dispersion caused by the transmitting single mode optical fiber can be compensated for, which actualizes high-speed communication.
Moreover, the wavelength division multiplexing system has recently progressed accompanying the increase in the transmission capacity. For example, in the case of compensating for the wavelength dispersion of the transmitting optical fiber for the wavelength of 1.3 μm by using a dispersion compensating optical fiber having a large negative wavelength dispersion and a positive dispersion slope, although the dispersion of one wavelength among wavelengths can be compensated for, the dispersions of other wavelengths are not sufficiently compensated for, and the transmission characteristics of the wavelengths far from the compensated wavelength are worsened.
Accordingly, a dispersion-slope-compensating and dispersion-compensating optical fiber having a W-type refractive index profile with a negative dispersion slope as shown in FIG. 6 (which is referred to as a “dispersion compensating optical fiber with a W-type profile”) was developed. In FIG. 6, a center core portion 1, a side core portion 2, and a cladding portion 4 are shown. In the dispersion compensating optical fiber with the W-type profile, the dispersion slope can also be entirely compensated for by controlling the specific refractive index difference Δ1 of the center core portion 1 relative to the cladding portion 4, the specific refractive index difference Δ2 of the side core portion 2 relative to the cladding portion 4, and the ratio of the radius 1 of the center core portion 1 relative to the radius b of the side core portion 2.
The dispersion-slope-compensating and dispersion-compensating optical fiber can compensate for the wavelength dispersion and the dispersion slope by rendering it into a cable for a transmission line or by inserting it as a small module into the receiving side or the transmitting side of an existing transmission line.
However, a conventional dispersion compensating optical fiber has a structure in which the specific refractive index difference Δ1 of the center core portion increases and the specific refractive index difference Δ2 of the side core portion around the core portion decreases, and has a small core diameter, in order to increase the absolute value of the wavelength dispersion per unit length while having a refractive index distribution profile as shown in FIG. 6. FIG. 7 shows the relationship between the dispersion slope and the wavelength dispersion when the value of b/a is changed with setting Δ1 to 1.8% and Δ2 to −0.4%.
In FIG. 7, a dotted line shows 100% of the dispersion slope compensation ratio, which is the desirable value of the dispersion slope compensation ratio. The dispersion slope compensation ratio is calculated by dividing the ratio of the dispersion slope of a dispersion compensating optical fiber relative to the dispersion slope of a transmitting single mode optical fiber by the ratio of the dispersion value of the dispersion compensating optical fiber relative to the dispersion value of the transmitting single mode optical fiber. Moreover, values of effective areas (Aeff) are also shown in FIG. 7. As shown in FIG. 7, although dispersion compensating optical fibers with the W-type profile can have desirable dispersion characteristics, the bending loss tends to increase and the effective areas tend to be small, which tends to cause nonlinear effects.
Although examples in which the dispersion slopes are compensated for by means of the dispersion compensating optical fibers each having a W-type profile are reported in TuG3 of OFC 2000 (Optical Fiber Communication Conference) and in C3-3-38 of the Institute of Electronics, Information and Communication Engineers in 2000, for example, effective areas (Aeff) of both of the dispersion compensating optical fibers are 18.4 μm2, which are not sufficiently large.
Moreover, in order to improve the bending loss and the dispersion slope characteristics, a dispersion-slope-compensating and dispersion-compensating optical fiber having segment-attached a W-type refractive index profile as shown in FIG. 1 has been developed. In FIG. 1, a center core portion 1, a core portion 2 disposed around the center core portion 1, a ring core portion disposed around the core portion 2, and a cladding portion 4 disposed around the ring core portion 3 are shown.
Although the present inventors reported, at 14C4-4 at the OECC 2000 (Optoelectronics and Communications Conference), one embodiment of this dispersion compensating optical fiber compensating the dispersion slope, which has an effective area (Aeff) enlarged to 21.0 μm2, the absolute value of the wavelength dispersion is 61.5 ps/nm/km, which is small. Therefore, it is required to increase the length of the fiber for producing a dispersion compensating optical fiber module by winding the fiber into a small coil. However, it is difficult to wind the fiber into the small coil and to miniaturize the module. When the length of the fiber used becomes longer, the cost required for producing the module increases.
The present inventors, Shimizu et al., reported, at C3-3-33 of the Institute of Electronics, Information and Communication Engineers in 2001, a dispersion compensating optical fiber module having low nonlinear properties, low losses, and superior dispersion slope compensating functions, which are not provided conventionally.
However, the dispersion compensating optical fiber has characteristics in which the microbending loss increases as the dispersion slope compensation ratio increases from low values to 100%. Moreover, the microbending loss tends to be caused by decreasing the dispersion values or increasing the effective area. In order to prevent the microbending loss, a method in which an optical fiber is fixed by a resin without using a reel (NFOEC (National Fiber Optic Engineers Conference) 2000, pp. 420 to 429).
An optical fiber generally includes a coating layer which is made from at least one ultraviolet light curable resin such as ultraviolet light curable urethane acrylate resin, and which is formed onto the surface of glass. Since the surface of the coating layer has a slight adhesive property (which is referred to as “surface tackiness”), which causes adhesion of portions of the optical fiber to other portions thereof, a dispersion compensating optical fiber module produced by winding a long optical fiber into a small coil has problems in that temperature characteristics are increased.
The adhesive property (surface tackiness) is defined as the degree to which optical fibers stick together. A specific example of a method for measuring the adhesive property is disclosed in Japanese Patent Application, First Publication No. Hei 10-62301, in which the tensile change of an optical fiber which is wound many times around a delivery roll so as to be overlapped is measured by rewinding the optical fiber with a constant tension.
Specifically, various situations are anticipated for environments in which a dispersion compensating optical fiber module is employed, and suitable operations of the dispersion compensating optical fiber module may be required from the low temperature region (down to −40° C.) to the high temperature region (up to +80° C.). A dispersion compensating optical fiber used for the dispersion compensating optical fiber module has a short fiber length, a large absolute value of the wavelength dispersion, and a large effective area, which enable miniaturization of the module. Therefore, the microbending loss of the dispersion compensating optical fiber is larger than those of conventional dispersion compensating optical fibers. As a result, problems are caused in which the loss at the low temperature region is increased by the microbending loss and the surface tackiness, when the module is produced by using the dispersion compensating optical fiber. Although the aforementioned problems are caused depending on characteristics of the dispersion compensating optical fiber, the problems are significant when the module is formed into a small coil in which portions of the optical fibers are adhered to other portions thereof.